Sunbirds' and squirrels' sweet treats help to set seed
Reproductive biology features strongly in this issue of the journal and we focus here on one of the papers that deals with this topic. Butea monosperma is a leguminous tree, well-known in India because of its beautiful orange-red flowers and several useful products. The flowers are visited by birds but there is a question as to whether these are effective pollinators. To answer this and several related questions, Tandon et al., Delhi and Bangalore (pp. 715-723) have carried out an extensive study of the reproductive biology of B. monosperma. Here we concentrate mainly on pollination. Three species of bird were recorded as feeding on nectar, but only one, the purple sunbird (Nectarinia asiatica) was a legitimate collector. The other two species gained access to the nectar by making a hole in the calyx. The authors also noted legitimate collection of nectar by a mammal, the three-striped squirrel (Funambulus tristiatus), whereas the only insect observed to forage nectar via the holes in the calyx made by birds was the giant Asian bee. Pollen was presented only to the legitimate foragers of nectar that press down on the keel of the flower. Pollen could be observed on the heads of sunbirds and on the snouts and heads of squirrels as they retreated from the flowers and this pollen transferred easily to the stigma of the next visited flower. Thus, there are two effective pollination agents, a bird and a mammal, a very unusual situation that has been revealed by careful observation in the field. But how effective are they? In manual pollinations, the authors obtained 5 % fruit set in inbred and 22 % in outbred fertilizations. In the wild, fruit set is about 5 %, suggesting that pollinations are mainly inbred. This is borne out by the behaviour of the pollinators, which visit many flowers on the same tree.
Professor J.A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
